[FLINK-37562] Do not check all JM replicas in observer

[gyfora]

What is the purpose of the change

Change the observer to only verify that at least one JM is available before trying to query it.

Verifying this change

Adapted unit tests

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changes to the CustomResourceDescriptors: no
• Core observer or reconciler logic that is regularly executed: yes

[csviri]

LGTM